The present invention relates to the elastomer mixes used in the manufacture of tires for motor vehicles and in particular it relates to elastomer mixes selected for the preparation of a tire tread strip. The invention relates more specifically to tires having enhanced performance resulting from the use of the above mixes in the tire strip and the method for obtaining such tires.
It is known that motor vehicle tires comprise a toroidal-shaped carcass, to be mounted on a wheel rim, internally hollow and inflated with air under pressure to allow the operation of the tire. In the tire crown, that is, in a radially external position, the tire has a thick strip (tread strip or more simply "tread") of an appropriate elastomeric composition, hereinafter called a "mix", onto which a tread design is impressed.
It is also known that different characteristics of the tire's behavior and performance depend to a not negligible extent on the characteristics of the above tread mix: as an example, it is sufficient to mention resistance to abrasion, which determines length of the life of the tire, road-holding features on dry as well as wet surfaces, resistance to rolling, properties of energy dissipation and drivability.
A specific type of use to which tires used for heavy truck vehicles are subjected is that, distinguishable into two variants, commonly known to the experts as "ON" and "SUPER-ON", which substantially indicate the use of the tire on vehicles with a high loading capacity, which run prevalently (ON) or almost exclusively (SUPER-ON) at high operating speeds, on substantially rectilinear and flat roads, in good or better surface conditions, such as interstate highways.
These tires ape generally distinguished by a tread design of the so-called "ruled" type, that is comprising a plurality of continuous circumferential ribs, axially separated one from the other by longitudinal grooves, circumferentially continuous, with a zig-zag pattern, or of the type having small blocks with a high tractability.
With this type of operation the qualities of good road-holding under all surface conditions, of response to driving conditions, of low tearability, of low Polling resistance, and of resistance to wear under low severity conditions, such as those indeed presented by the above-mentioned types of roads, acquire particular importance.
Unfortunately several of these characteristics one substantially incompatible with one another, in the sense that mixes outstanding as regards some of them are generally poor as regards the others, such as, for example, mixes based on natural rubber which ape highly resistant to tear and have a low Polling resistance but frequently have an insufficient road-holding features and high abradability.
The addition in the recipe of these mixes of given quantities of specific elastomeric materials (for example, styrene-butadiene rubbers) on the one hand raises road-holding ability and resistance to abrasion but, at the same time, worsens rolling resistance and tearability under certain operating conditions.
In this way the expert is obliged to look for the best possible compromise between the different characteristics in view of all the possible features of performance required of a specific type of tire.
In order indeed to reach the best compromise in relation to that particular type of use, the current state of the art utilizes for the uses specified above different types of tread mix.
These types of mix use natural rubber as an elastomeric material, in combination with appropriate charges of other specific materials (lampblack, vulcanization regulators, anti-aging products, etc.) and can also contain other reinforcing charges in siliceous materials for the purpose of suitably changing certain of their characteristics.
Two characteristics of the tire that are very important for the safety of road and motorway transport are behavior on a dry surface and road-holding on a wet surface.
These two characteristics determine the possibility of controlling the direction of the motor vehicle, preventing dangerous swerves, even under emergency conditions due, say, to a sudden obstacle to be avoided.
These characteristics are measured by means of the assessment of the behavior of the tire mounted on the vehicle, driven along a specific type of route.
The assessment can be carried out on the basis of a feeling on the part of the test driver or by means of the measurement of time and speed.
On the basis of tests carried out by the Applicant with radial tubeless truck vehicle tires, size 315/80 R 22.5, provided with different types of tread mixes, the behavior on dry surfaces and road-holding features on wet surfaces have been shown to be mathematically correlatable with the dynamic properties of the tread mixes, expressed by the parameters: E'=elastic modulus (which measures the energy conserved and recovered during a cyclic deformation); E"=viscous modulus (which measures the energy dissipated as heat during a cyclic deformation); tang .delta.=E"/E'.
Another characteristic of fundamental importance in truck vehicle tires is the rolling resistance, that is the energy dissipated during rolling. This property, as is known to experts in the field, affects not only fuel consumption but also the useful life of the tire.
In fact the higher the Polling resistance the higher the tire's operating temperature and the quicker are those processes of chemical and physical decay of the materials which lead to the tire having to be discarded.
The Applicant, carrying out tests still with the above-mentioned tires size 315/80 R 22.5 and using different tread mixes has found a correlation between the dynamic properties of the tread mixes and the rolling resistance, which can both be expressed in relation to tang .delta..